The ever-increasing processing speed of the central processing unit in electronic computers is requiring output printing terminals of a higher speed capacity, and there have been proposed various systems for this purpose. The present invention relates to a dot printer for printing characters and the like with a dot matrix.
Such dot matrix printer is represented, for example, by a wire dot printer, a thermal printer and so on. In a wire dot printer, for example, the printing of dots is performed as shown in FIG. 1 by a print head 3 which is provided in front of a platen 1 and a paper sheet 2 and which presses an ink ribbon 6 against said paper sheet 2 while moving parallel thereto along the guide shafts 4, 5. As shown in FIG. 2, the above-mentioned print head 3 is provided, for example, with seven wires 7 arranged on a vertical line at the front end thereof and also is provided with seven magnets 8 at the rear end thereof for pushing said wires 7 to the printing position. The timing of dot printing, or, driving said wires 7 to said printing position, is determined by timing signals obtained by optically or magnetically detecting the slits 10 on a slit plate 9 mounted on said print head 3. FIG. 3 shows an example of slit detecting means in which the spacing between the slits 10 on said slit plate 9 corresponds to the spacing between the dot lines. Thus, a signal generated by a slit detecting element 12 incorporating a light receiving element 11 indicates that the print head 3 is advanced to a next dot printing position and activates the magnets 8 to advance the corresponding wires 7 to the printing surface.
Conventional wire dot printers have insufficient print quality due to a wider spacing of dots, as exemplified in FIG. 4 which shows the printing of character "E" and "A" by a 5 .times. 7 dot matrix.
For preventing such poor quality printing it has been proposed to print additional dots between dot lines. Such printing can be achieved for example by means of two wire lines which are mutually displaced by a half pitch and one of which is adapted to print the dots on the matrix while the other is adapted to print additional dots in positions displaced by a half pitch from aforementioned dots. Such method, however, requires a larger print head due to the presence of two wire lines and correspondingly a larger number of magnets for driving said wires. In order to prevent the above-mentioned drawbacks there is also known a dot printer with a single wire line for printing, wherein the dots on the matrix positions are printed by a timing signal generated for example by a slit plate and the additional intermediate dots are printed at a predetermined time after a timing signal, said predetermined time corresponding to one half of the period between two timing signals. Such printer, however, is still defective as the intermediate dots become not exactly positioned in the center between the dot lines on the matrix, thus deleteriously affecting the print quality, when the speed of movement of the print head has a fluctuation due to the characteristics of the spacing drive mechanism and the mass of said print head, since the timing of printing intermediate dots is fixed at a predetermined time after the timing signal.